Hearing in Old Age.

 Hearing in Old Age

PKGhatak, MD

If one has passed 65 years of age, and for the first time hears someone telling he/she is short of hearing. The person feels a bit startled and annoyed. But the same person did not feel the same way when had to get a pair of reading glasses. Dealing with the deficit in hearing is different due to social stigmata. And wearing a hearing aid is a no-no.

The Latin term of old age hearing deficiency is Presbycusis. In the USA, one in three are over the age of 65 yrs. and nearly one in two over 75 yrs. has difficulty in hearing.

Persistent exposure to loud noise damages the delicate hearing organ and results in selective deafness for the frequency of sound overexposed to and once damaged it fails to grow back. In normal conversation the sound waves are like ripples on a slow flowing stream, noise in the work environment is like winds in winter storms and the noise that hits the ears in live concerts is like tsunami waves. The delicate hearing organs are not designed for mega loudspeakers and music synthesizers and the sound receptors simply wither away.

A short account of the hearing should make this clear.

The receptors of sound are called Organ of Corti. The Organ of Corti is located in the inner ear called Cochlea because of its resemblance to a snail. Inside the bony cochlea, all along the entire length, has another canal made of the membrane. Both the bony and membranous canals are filled with fluids, however, the fluid of the two canals is different in composition. The fluid of the bony canal is called Perilymph and has a high concentration of Sodium, the fluid in the membranous canal is called Endolymph which has higher Potassium levels.

Organ of Corti.

On the base layer of the membranous canal called the Basement Membrane, several groups of Hair Cells are situated on it. The hair cells are arranged in 4 layers of cells and bunched together in groups. Nerve fiber is attached to the lower pole of each hair cell. The tall hair cells have hair like projections on the top which are embedded in the top layer of the membranous canal. When the basement membrane moves up and down, the hair bends and stretches. This movement triggers the opening of pores of the hair cells and allows Sodium from the outer canal to enter the hair cells, triggering an electrical impulse that the nerve fibers carry to the hearing center in the brain.

The hair cells at the beginning of the canal respond to high frequency sounds and the hair cells on the far end of the spiral canal respond to low frequency sounds. And in between the hair cells respond to sound from high to low frequencies.

The three outer layers of hair cells receive signals from the brain and function as sound amplifiers to the outgoing impulse to the brain like transistors in radios.

The tension of the basement membrane has regional variation based on the thickness and composition of tissues. The region of the high tension area of the basement membrane vibrates to high frequency sounds and the low frequency sounds move the low tension area.

Functions of the other parts of the ear.

The outer ear, the pinna, collects sound waves, concentrates the sound waves and sends them down the ear canal. The eardrum vibrates and the vibration is transmitted to the stirrup-like tiny piece of bone attached to the inner side of the eardrum. The final of the three-piece of bone seats perfectly on the opening of the bony canal called the Oval Window and tissues around the bone make the joint airtight. Since the surface area of the oval window is 1/20th of the eardrum, the sound wave is magnified 20 times at the oval window. The waves are transmitted to the perilymph. The movement of the perilymph moves the endolymph in the membranous canal. As the endolymph moves, the basement membrane moves up and down generating nerve impulses. The 2 and 1/3 turns of the cochlea with progressive narrowing, amplify sounds further as the sound waves move to the far end of the cochlea.

Cause of hearing loss in elderly.

Like any organ, age takes its toll, but misuse and abuse accelerate the degenerative changes leading to loss of function. And in this modern age, humans are surrounded by air pollution, water pollution, light and sound pollution. The OSA regulations aim to protect the employees but the compliance is not universal and particularly true for companies with seasonal employees and mom and pop shops. And those weekend homeowners using lawnmowers, chain saws or leaf-blowers are likely to have a hearing deficit.

Genes are blamed for most maladies and so are presbycusis also. But the blame lies squarely on misuse.

The deficit due to presbycusis is detected on both ears. In an acquired illness deafness is unilateral.

The world around us.

It is interesting to survey the living world around us and note the evolution of hearing.

Plants.

Some claim plants can hear us talking to them, which makes plants grow healthier and faster. But plants have no nervous system at all. So, it is up to future scientists to find that truth.

The unicellular organism onward up to the worm:

The perception of sound in these organisms is through the surface in contact with the environment. The worms have nerve innervation of the segmental body, the nerve ends detect ground vibrations.

Insects:

The majority of insects can hear. An insect's so-called ear is an open tube, the opening of the tube is covered with overlapping cuticles. The location of sound receptors varies from one species to the other. It can be located in the abdomen, thorax, or head. Insects can hear a wide range of sounds.

Amphibians:

Salamander has primitive hearing organs inside the head and a tiny opening, one on each side admitting sound waves to the inner ear.  Frogs can hear in water and on land.

Fish:

Fish have the most elaborate hearing system. Fish have well developed inner ears on each side of the head. In addition, the lateral line of fish has nerves connected with receptors called cilia. Cilia move with the vibration of water. In some fish, the swim bladder has projections reaching the inner ears and act as sound receptors. The head of some bony fish acts as a receptor of wave movement. In some bony fish, the pectoral fine bones function as additional water vibration receptors. Fish, being underwater animals, hear only low frequency sounds.

Reptiles:

Snakes.  Snakes have no external or middle ears but have well developed internal ears. A special bone in the head connected with other bones by ligaments acts as the sound receiver. Snakes also feel ground vibrations with the body.

Lizards have the middle and inner ears but not the external ear. Ground burrowing lizards have a hearing system like snakes.

Birds:

Birds have ears, but not the pinna. The outside opening of the external canal is covered with special feathers without any burs. In vultures and condors, the external ear openings are easily visible. Songbirds have a wide range of hearing.

Mammals:

Whales and dolphins used to be land mammals, later they went back to the water. The pinna of aquatic mammals has disappeared, and the ear canals are filled with either wax or oil to prevent water entry but conduct water vibration well. Water being dense and not easily compressible, the sounds travel far and wide.

Dolphins can hear in water and out of water. In water, they hear through the vibration receptors of the lower jaw bone. When out of the water, the dolphins hear like land animals, the air enters the ears through two small openings on the side of the head. In addition, dolphins have an echolocation box on the head, a specialized receptor for ultrasonic sounds, and as the ultrasounds are received the dolphins reconstitute the ultrasound in the form of images of the prey and the immediate surroundings.

Whales:

Baleen whales (toothless) have two small external openings for the ear canal and the canal is filled with wax. The inner hearing organ is like land animals.

Toothed whales have no openings for their ears. They have a specialized structure on the lower jaw bone which acts as a receiver of sound waves. Whales can hear from very long distances but only low frequency sounds.

Land animals.

Dogs: Dog is the hearing champion among territorial mammals. A dog can move pinna in any direction by using some of the 30 muscles. A dog's hearing range is 20 to 40,000 cycles per second (Hertz); far beyond the human hearing range of 20 to 4,000 Hertz.

A look at the range of hearing of some animals.

Moth – up to 300,000 Hertz

Bullfrog - 50 – 4,000 Hertz

Owl – 200 – 12,000 Hertz

Songbird - 1000 – 8000 Hertz

Dolphin - 75 – 15,000 Hertz

Beluga whale -1000 -12,000 Hertz

Human - 64 – 23,000 Hertz

Dog - 20 – 45,000 Hertz.

The champion of hearing is the Moth.

Presbycusis comes with old age but by using ear covers in a loud noisy environment the hearing deficit can be delayed, or completely prevented.

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